The present invention relates to a method and system for controlling the ignition timing of internal combustion engines whereby knocking is detected from the occurrence of vibrations, sound or the like caused inside and outside the cylinders of an engine due to the pressure therein and the ignition timing of the engine is adjusted to maintain a desired knock intensity.
The ignition timing of an engine must be controlled in accordance with the engine operating conditions so as to ensure the optimum operation of the engine. Ignition timing control systems known in the art are generally so designed that the ignition timing is controlled in accordance with the engine conditions represented by the engine speed detected by the centrifugal advance mechanism and the intake negative pressure detected by the vacuum advance mechanism.
It is known in the art that the best way from the standpoint of engine efficiency and fuel consumption is to effect the ignition at a position near a so-called minimum advance for best torque or MBT, and the ignition timing must be adjusted to the MBT in accordance with the engine conditions.
However, if the ignition timing is advance under certain engine conditions, knocking will be caused and stable operation of the engine will not be ensured. Generally, it is known that there is a close correlation between the ignition timing and the cylinder pressure so that when a mixture is exploded, if no knock occurs, no harmonic component (usually a frequency component in the range 5 to 10 KHz) will be superimposed on the cylinder pressure, whereas if knock occurs, such harmonic pressure variation will be caused. This results in the generation of vibration or sound outside the cylinder. Many different types of so-called knocking feedback ignition system have been proposed in which such vibration or sound is detected to control the ignition timing.
The known systems of this type are such that the ignition timing is controlled in response to every combustion phenomenon, that is, the presence or absence of knock is detected in response to each explosion stroke of the respective cylinders and the resulting detection signal is utilized in such a manner that as for example, when knock occurs the ignition timing is retarded by a predetermined angle, and when it is determined that there is no knock the ignition timing is advanced by a predetermined angle.
However, the use of feedback in this type of system has a great disadvantage that in the case of a multicylinder engine, due to the difference in intake lines for fuel distribution, mixture distribution, etc., among the cylinders as well as the difference in conditions (deposit, compression ratio, etc.) among the combustion chambers, variation in combustion among the cylinders will be increased, thus increasing the hunting range of the ignition timing and thereby causing surging or slugging.